The present invention relates to a speed control method for an injection molding machine.
Conventionally, to injection-mold a molten resin by using an injection molding machine, the molten resin is filled into the cavity of a mold at a high speed. After the molten resin is filled, a pressure is applied to the resin in the cavity of the mold, thereby molding the resin. This injection control will be described more practically with reference to FIG. 9. Referring to FIG. 9, a position S of the injection screw and a time t are plotted along the axis of abscissa, and a forward speed V and a pressure P are plotted along the axis of ordinate.
Injection control is performed by a filling step S1 of filling a molten resin into a mold cavity at a high speed through several sub-steps, and a dwell pressure application step S2 of applying several stages of pressures to the resin in the mold cavity, after the molten resin is filled, thereby molding the resin. In most cases, in the filling step S1, the pressure regulating valve of a hydraulic circuit is set at a high pressure. The opening degree of a flow control valve is set to change in accordance with the lapse of time or in accordance with the stroke position of the injection screw, so that the speed is changed in a plurality of stages V1, V2, and V3 with reference to the lapse time since the start of injection or to the forward position of the injection screw. The injection speed of the piston of the injection cylinder, i.e., of the injection screw, is controlled by adjusting the flow control valve. The molten resin is filled into the cavity at a speed high enough not to trap air in the resin in the cavity. After the molten resin is filled in the cavity, the dwell pressure application step S2 is performed, wherein the flow control valve is fixed at a comparatively small opening degree and the hydraulic pressure is regulated by the pressure regulating valve. In the dwell pressure application step S2, the opening degree of the pressure regulating valve is changed so that a predetermined pressure can be applied to the molten resin, filled in the cavity, in accordance with the lapse of time. The dwell pressure application step S2 prevents shrinkage of the resin so that an error will not occur between the shape and size of the product and the shape and size of the cavity when the molten resin is cooled in the cavity, and prevents a large residual stress from being generated in the product.
Generally, in injection process control of the filling step, the target control values of a plurality of stages of injection speeds are switched when the detection value of the injection screw coincides with a preset injection speed position, as described above. FIG. 10 shows an example of this switching operation. In FIG. 10, changes in injection speed V and in pressure P, obtained when switching control between three-stage preset speed values V1, V2, and V3 (V1&gt;V2&gt;V3) is performed in the filling step S1, is shown. In this manner, although changing or switching between a plurality of stages of injection speeds in the filling step is performed in accordance with the lapse of time very rarely, it is mostly performed as positional switching at every change/switching points, in which a switching operation is performed when the injection screw reaches a preset position.
A check valve for preventing a reverse flow of the molten resin in front of the injection screw toward the injection screw during injection does not operate at the same timing in each shot but operates at slightly different timings from one shot to another. Therefore, when the check valve is closed with a delay, the filling amount of the resin into the mold cavity, as compared to that obtained when the check valve is closed early, at the same screw position is decreased by an amount corresponding to an increase in reverse flow.
At a V - P switching point where a shift occurs from the filling step to the dwell pressure application step, a preset pressure switching operation is generally performed. Thus, in a shot in which the check valve is closed with a delay, the injection screw position (to be referred to as a V - P switching position hereinafter) where a V - P switching operation is performed becomes close to the forward limit of the screw. As a result, an area controlled by the preset speed of the final stage of the filling step is elongated. For example, in FIG. 10, a final-stage injection area S.sub.B of a shot B in which the check valve is closed with a delay becomes larger than a final-stage injection area S.sub.A of a shot A in which the check valve is closed early.
Especially, when a resin material, e.g., nylon, which has a low melt viscosity with a high temperature dependency, is molded, the check valve closing characteristics vary from one shot to another more largely than with other resin materials. As a result, the V - P switching position also varies, and the width of variations of the area controlled by the preset speed of the final stage becomes large, thereby increasing the width of variations of the final-stage injection time. FIG. 11 shows a state wherein the final-stage injection speed time in the conventional position switching operation varies from one shot to another due to variations in closing characteristics of the check valve. In this manner, in a shot in which the check valve is closed with a delay, the injection time is prolonged, and the amount of resin cooled in the mold cavity is increased by an increase in injection time. Thus, the flow resistance in the mold cavity is increased, so that a filling amount into the cavity is short, causing a defective molded article having an insufficient volume.
As described above, conventionally, variations in check valve closing characteristics that differ from one shot to another cause non-uniformity in filling amount of the resin in the cavity, leading to generation of a sink mark on the surface of the molded article, generation of a shrinkage void in the molded article, variations in denseness, variations in weight of the molded articles, and variations in quality of the molded articles accompanying the variations in weight of the molded articles.